Extensive research is being conducted to provide various Quality of Service (QoS) features with high data rates in the advanced fourth-generation (4G) communication systems. The 4G communication systems are evolving to provide high mobility and QoS features in wireless communication systems such as Local Area Network (LAN) systems and Metropolitan Area Network (MAN) systems.
In general, because wireless communication systems consider the mobility of a Mobile Station (MS), the power consumption of the MS is evaluated as a key performance index of the entire system. Thus, a wireless communication system standard such as Institute of Electrical and Electronics Engineers (IEEE) 802.16 defines an active mode and a sleep mode of an MS to minimize the power consumption of the MS.
FIGS. 1A and 1B illustrate a sleep mode operation defined in the conventional IEEE 802.16 system.
Referring to FIG. 1A, an MS receives a mode transition allowance from a Base Station (BS) to transition to a sleep mode in the IEEE 802.16e system. Herein, the BS may buffer or drop packet data to be transmitted, while allowing the transition of the MS to the sleep mode. Also, during a listening window or a listening interval of the MS, the BS informs the MS whether there are data to be transmitted to the MS, and the MS wakes up from the sleep mode and determines whether there are data to be transmitted to the MS. In FIGS. 1A and 1B, TRF-IND (MOBile TRaFfic INDicator (MOB_TRF-IND) messages 100, 102 and 104 are transmitted to inform whether there are packet data to be transmitted. Herein, the TRF-IND messages 100 and 102 containing ‘Negative Traffic Indication’ indicate the absence of packet data, and the TRF-IND message 104 containing ‘Positive Traffic Indication’ indicates the presence of packet data.
If it is determined during the listening interval or the listening window that there are data to be transmitted to the MS, the MS transitions to the active mode and receives data from the BS. Thereafter, in order to transition from the active mode to the sleep mode, the MS transmits/receives SleeP-REQuest/ReSPonse (SLP-REQ/RSP) to/from the BS. This may cause a signaling overhead.
In order to prevent the signaling overhead, the MS may not transition to the active mode even in the case of presence of traffic, as illustrated in FIG. 1B. In other words, there is a limitation in that the transition between the active mode and the sleep mode must be made by MOBile SLeeP-REQuest/ReSPonse (MOB_SLP-REQ/RSP) signaling. Accordingly, during the sleep mode, the MS repeats a sleep state and an awake state in a sleep cycle and receives data.
Referring to FIG. 1B, during the listening window or the listening interval in a sleep state, the MS receives TRF-IND messages 100, 102, 104, 110 and 112 from the BS. Specifically, the MS receives the TRF-IND message 100 containing ‘Negative Traffic Indication’ during the listening window in the initial sleep cycle and increases the next sleep cycle by two times. After the lapse of the next sleep cycle, the MS receives the TRF-IND message 102 containing ‘Negative Traffic Indication’ and sets the next sleep cycle to two times the current sleep cycle. After the lapse of the sleep cycle, the MS receives the TRF-IND message 104 containing ‘Positive Traffic Indication’, receives downlink data during the listening window for the TRF-IND message 104, and resets the next sleep cycle to the length of the initial sleep cycle. Thereafter, during the listening window after the lapse of the initial sleep cycle, the MS receives the TRF-IND message 110 containing ‘Negative Traffic Indication’ and increases the sleep cycle by two times. Thereafter, during the listening window after the lapse of the increased sleep cycle, the MS receives the TRF-IND message 112 containing ‘Negative Traffic Indication’ and again increases the sleep cycle by two times.
If the MS normally receives the traffic indication information through the TRF-IND message transmitted by the BS, sleep cycle synchronization is maintained between the MS and the BS. On the other hand, if the MS fails to receive the TRF-IND message due to channel state degradation, the MS cannot know whether the BS has transmitted ‘Positive Traffic Indication’ or ‘Negative Traffic Indication’. Consequently, the MS cannot correctly set the length of the next sleep cycle. This causes sleep cycle desynchronization between the MS and the BS.
FIG. 2 illustrates sleep cycle desynchronization in the conventional IEEE 802.16 system.
Referring to FIG. 2, an MS normally receives TRF-IND (MOB_TRF-IND) messages 200 and 202 containing ‘Negative Traffic Indication’ during the listening window of a sleep cycle, and fails to receive a TRF-IND message 204 containing ‘Negative Traffic Indication’ during the listening window after the lapse of the next sleep cycle. Herein, a BS recognizes the listening window of the next sleep cycle as a time point t1 (212). However, because the MS has failed to receive the TRF-IND message 204, the MS cannot determine whether to reset the next sleep cycle to the initial sleep cycle according to ‘Positive Traffic Indication’ or to increase the next sleep cycle by two times according to ‘Negative Traffic Indication’. The start time point of a listening window of an actual sleep cycle is the time point t1 (212). However, if the MS resets the next sleep cycle to the initial sleep cycle, it causes desynchronization of the listening window of a sleep cycle. Thus, sleep cycle desynchronization may occur if the MS fails to receive the TRF-IND message in the wireless communication system.
In order to solve the problem of sleep cycle desynchronization, the MS transmits a TRF-IND request message to the BS if the MS fails to receive not only the TRF-IND message but also downlink traffic until the end of the listening window. For example, the TRF-IND request message may be an AAI (Advanced Air Interface)_TRF-IND-REQ message shown in Table 1 below.
TABLE 1SizeField(bit)Value/DescriptionAAI-TRF_IND-REQmessage_format( ) {Control Message Type8This indicates type ofcontrol message is AAI-TRF_IND-REQ message
When receiving the TRF-IND-REQ message shown in Table 1, the BS transmits a TRF-IND-RSP message to the MS on the basis of ‘Positive Traffic Indication’ or ‘Negative Traffic Indication’ that is a traffic indication given to the MS through the AAI_TRF-IND message. For example, the TRF-IND-RSP message may be an AAI_TRF-IND-RSP message shown in Table 2 below.
TABLE 2Size Field(bit)Value/DescriptionAAI-TRF_IND-RSPmessage_format( ) {Frame_Number10The least significant 10 bits ofthe frame number in whichincoming LW will start0~1023Sleep Cycle Length10The length of sleep Cyclewhich contains the nextscheduled Listening Window.If the AMS receives thenegative traffic indicationduring a the next scheduledListening Window, the(current) Sleep Cycle shall beset to this value at that time.Value: 0~1023Sleep Cycle Length = Value + 1
As can be seen from the AAI_TRF-IND-RSP message of Table 2, the BS notifies the MS of the frame number for a next scheduled listening window. Also, if ‘Negative Traffic Indication’ occurs in the listening window of the notified frame number, the BS notifies the length of a doubled sleep cycle.
For reference, a sleep cycle includes a listening window and a sleep window. The listening window is located at the front of the sleep cycle. Thus, if ‘Positive Traffic Indication’ is received in the current listening window, the current sleep cycle including the current listening window is reset to the initial sleep cycle size. Accordingly, the next scheduled listening window starts at a relatively near point, that is, the current sleep cycle is immediately after the initial sleep cycle. On the other hand, if ‘Negative Traffic Indication’ is received, the current sleep cycle increases by two times the previous sleep cycle. Accordingly, the next scheduled listening window starts at a relatively distant point.
The TRF-IND message may contain emergency alert information, an example of which is shown in Table 3 below.
TABLE 3SizeField(bit)Value/DescriptionEmergency Alert1Used to indicate the presence ofIndicationemergency information for supportingthe emergency alert service0b0: there is no emergency information0b1: there is emergency information
When an emergency alert indication of Table 3 is set to 1, it means that an emergency alert service-related message will be transmitted. That is, when the emergency alert indication is set to 1, it means that the MS does not enter the sleep state in the sleep window but maintains the awake state to receive an emergency alert service-related message.
As described above, an emergency alert service is considered in the TRF-IND message, whereas an emergency alert service is not considered in the exchange of the TRF-IND-REQ/RSP messages. In other words, if the MS loses the TRF-IND message, the MS can only inquire of the BS what the MS's traffic indicator is, but cannot determine whether there is an emergency alert service-related message.